1. Field of the Invention
The present invention relates to an overcurrent protection apparatus that detects an overcurrent of series regulators of plural stages and protect the series regulators from the overcurrent, and an electronic apparatus.
2. Description of the Related Art
A series regulator steps down, for example, a battery voltage to a stable voltage and outputs the stepped-down voltage. However, a loss caused by the output transistor disposed in the regulator is large, so that the overcurrent protection apparatus employs a construction in which the loss is dispersed by additionally disposing a regulator in a former stage to construct the regulator in two stages or by disposing means for stepping down voltage at the former stage.
A power supply system described in Japanese Unexamined Patent Publication JP-A 2002-297249 includes a series regulator and a power consuming part disposed on a direct current passage between a direct current power source connected to the power supply system and the input side of the series regulator. The power supply system disperses the power loss, that is, loss of the whole circuit into the series regulator and the power consuming part, thereby reducing the heat produced by the series regulator.
A direct current stabilizing power supply system described in JP-A 2003-241842 includes: a voltage step-down part for stepping-down a first direct current voltage to be inputted to a second direct current voltage; a series type regulator, that is, series regulator for stepping-down the second direct current voltage to a third direct current voltage; and a comparison circuit for determining whether or not the first direct current voltage is a specified reference value or less. The direct current stabilizing power supply system disperses loss into the voltage step-down part and the series type regulator, and when the first direct current voltage is the specified reference value or less, the comparison circuit controls the voltage step-down part in such a way that voltages at the input terminal and the output terminal of the voltage step-down part are brought into the same voltage. With this, even when an input voltage becomes low, the direct current stabilizing power supply system can keep a constant output voltage.
A power supply circuit system described in JP-A 2006-127253 includes regulator circuit sections of N stages connected in series, and each of the regulator circuit sections performs a direct current voltage conversion. The power supply circuit system can reduce the allowable loss of each regulator circuit section to 1/N and hence can reduce the heat produced by each regulator circuit section.
These systems disperse the loss into the power consuming part, the voltage step-down part, or the plural series regulators. In the case of dispersing the loss into the plural series regulators, to prevent each series regulator from being broken by overcurrent, each series regulator is provided with an overcurrent protection circuit. The overcurrent protection circuit is a circuit that detects a current value of current flowing through the series regulator and which intercepts the current flowing through the series regulator when the detected current value is larger than a specified reference current value showing overcurrent.
However, in the case where the reference current value by which the overcurrent protection circuit of the series regulator of the former stage determines that a detected current value is overcurrent is smaller than a reference current value by which the overcurrent protection circuit of the series regulator of the latter stage determines that a detected current value is overcurrent, when the output side of the series regulator of the latter stage is grounded to cause overcurrent, the overcurrent protection circuit of the series regulator of the former stage operates before the overcurrent protection circuit of the series regulator of the latter stage operates. The series regulator of the latter stage is brought into a conducting state until the overcurrent protection circuit of the series regulator of the former stage operates, whereby the electric potential of the output side of the series regulator of the former stage drops to a ground level. At this time, the same overcurrent flows through the series regulator of the former stage and the series regulator of the latter stage, and since a voltage drop of the series regulator of the former stage is large, there is the possibility that the loss will be concentrated on the series regulator of the former stage to break the element of the series regulator of the former stage because the loss is over the allowable loss of the series regulator.
Further, in the case where the reference current value of the series regulator of the former stage is excessively large, when the output side of the series regulator of the former stage is grounded to cause overcurrent, there is the possibility that until the overcurrent reaches the reference current value, the overcurrent protection circuit will not operate to cause a large voltage drop in the series regulator of the former stage, which causes a break in the element of the series regulator of the former stage because the loss is over the allowable loss of the series regulator.